One of the common approaches to the circuitry of an inverter drive for powering an AC motor at adjustable frequency provides power to the motor with the voltage proportional to frequency, often referred to as a constant volts per Hertz method. By applying the voltage proportional to frequency, an attempt is made to cause the excitation current of the motor and therefore the rotating motor flux to be constant at all frequencies, thereby making full torque available to the load.
However, load current causes an impedance voltage drop in the stator windings of the motor, and the resulting excitation is therefore somewhat dependent upon the load of the motor applied to its rotor. To avoid problems with this variation in excitation, the volts per Hertz is generally fixed at the full, rated load level. If the load is less than rated, the impedance drop will similarly be less than rated, and the excitation will be greater than desired, resulting in saturation of the motor core, and attendant excessive energy losses and heating of the motor.
An additional problem exists at low speeds. At relatively high speeds, or frequencies, the stator impedance is predominately inductive, and the constant volts per Hertz energization is reasonably effective in accomplishing the goal of constant flux. At very low frequencies, however, the resistance portion of the stator impedance becomes larger that the inductive reactance, and constant volts per Hertz results in a drop in motor flux, and a similar drop in available load torque.
This low speed problem is solved by increasing the voltage at low speeds, with a technique known as "low speed boost". An adjustment for the "boost" is common on many available inverter drives. Again, to allow good regulation with load variations, the voltage boost is set for full load, and motor losses tend to increase to an excessive level at light loads.
Constant volts per Hertz AC inverter drives usually do not have any means of limiting motor torque for motor and control protection, and generally "trip out" or disable the drive in the presence of excessive load.
More sophisticated AC inverter drives measure the actual speed of the motor shaft, the stator winding currents, and the applied frequency, and perform complex calculations to establish the proper excitation at all conditions of speed and load. These drives are commonly known as "vector drives" and are very successful in accomplishing their goals. This performance comes at a higher price, partly because of the need to measure stator winding currents, and partly because of the complexity of compensating for temperature variation in motor resistance in the calculation for determining the proper motor excitation.